The area of filtration and its tremendous market potential has led to the development of several new types of porous materials with precisely controlled properties. These porous materials can act as filters in such varied applications as hot gases, molten metal, and waste water treatment.
Filtration applications are becoming more demanding on the filter's material requirements. For example, the food industry uses acidic materials during food processing which require corrosive-resistant filters. Molten metal filters must be able to withstand high temperatures. For applications in such severe environments, ceramics are becoming increasingly popular as filter materials, due to their exceptional high temperature stability, and superior resistance to chemical attack. One material that has these desirable properties is aluminum borate (Al.sub.18 B.sub.4 O.sub.33). However, aluminum borate has not heretofore been fabricated with the porosity characteristics necessary for applicability as a filter medium.
U.S. Pat. Nos. 3,856,702 and 3,856,705, both by McArthur, describe crystalline aluminum borate catalyst supports produced by pre-calcining shaped composites of alumina (Al.sub.2 O.sub.3) and boria (B.sub.2 O.sub.3) at temperatures between about 1250.degree. F. and about 2600.degree. F. (675.degree. C. to 1425.degree. C.). The alumina and boria powders are intimately mixed and formed into a desired shape prior to calcining. The resulting sintered body preferably has a porosity of about 0.2 ml/gm to 0.8 ml/gm.
U.S. Pat. No. 4,804,646 to Ray discloses a shaped aluminum borate product having a modulus of rupture of at least 45,000 psi (310 MPa) and a density of approximately 2.9 g/cm.sup.3. The product is formed by reacting alumina and boria at a temperature of at least 800.degree. C., grinding the reaction product, pressing the resulting shaped form and sintering the shaped particulate at a temperature of from 800.degree. C. to 1400.degree. C. under a pressure of 2500 psi to 3500 psi.
U.S. Pat. No. 4,789,422 to Misra, discloses a process for producing aluminum borate fibers by reacting aluminum salt in a solution with aluminum borate and heating the gel to form aluminum borate fibers. The fibers have a length from about 20 to 200 micrometers and a diameter of from about 1 to 10 micrometers.
U.S. Pat. No. 3,795,524 to Sowman, discloses a process for producing aluminum borate and aluminum borosilicate articles using aqueous-based processes. It is disclosed that an especially useful raw material which can be used to prepare a precursor aqueous solution is basic aluminum acetate stabilized with boric acid. In this process, the dilute liquid is concentrated or "viscofied" in order to convert it to a viscous or syrupy fluid concentrate which will readily gel when the concentrate is formed into a shaped article and dehydrated. It is disclosed that transparent fibers 10 to 20 feet in length are formed.
It would be useful to have a process for forming aluminum borate fibers and aluminum borate ceramics which is simplified and economically advantageous. It would further be useful to have a process for producing aluminum borate ceramics with a high degree of porosity and a tightly controlled pore size distribution.